The invention relates to a plant with high-temperature fuel cells and a multi-component sleeve for a cell stack which includes axially directed chambers for an afterburning process.
A plant of this kind includes a fuel cell battery such as is known from EP 1 037 296 A1. The fuel cells form a cylindrical cell stack with afterburning being carried out at the periphery of the cell stack in axially directed chambers, with exhaust gas at around 1000° C. arising. Entry points are present between the afterburning chambers through which the preheated process air is supplied to the cells for power delivering electro-chemical reactions. A fuel gas is distributed to the cells via an axial passage at the center of the stack.
Each fuel cell includes two parts, namely a PEN element and a disc-like interconnector. The PEN element which consists of at least three layers, namely P (cathode, i.e. positive electrode), E (electrolyte) and N (anode), is an electro-chemically active element with which electro-chemical reactions can be carried out at 700-900° C. The PEN element has the form of a thin circular disc which consists of a layer-like solid electrolyte and two electrodes P and N respectively applied by coating. The disc-like interconnector separates a space for the air from a space for the fuel gas. It has an architecture with a relief-like profile by which a flow of the fuel gas from a central entry point along the PEN element to the periphery is made possible, on the one hand. The flow of the air, on the other hand, is directed by the architecture of the interconnector from entry points into a central region and from there along the PEN element back to the periphery. At the periphery of the cell stack there are discretely arranged openings for the entry and exit of the air and fuel gas, respectively.
The afterburning chambers are surrounded by layers or shells of heat insulating material which, together with the chambers, forms a multi-component sleeve of the cell stack. A reformer is arranged within the sleeve and directly below the cell stack in which the fuel gas is catalytically converted at least partly into CO and H2. In this connection oxygen (air) is mixed with the fuel gas so that a partial oxidation results. The generation of CO and H2 is made possible by the partial oxidation and the heat delivered for the endothermic reactions.
The multi-component sleeve is formed as a heat insulating system. Its heat insulating function plays the role of an external recuperator. Instead of the air required in the cells for the electro-chemical processes first having to be preheated in a separate external recuperator, the air, which is initially cold or only slightly preheated, is used as a heat sink, in that the heat flowing away from the cell stack is partly taken up by the air in the sleeve and is returned again to the reaction sites.
The sleeve is of multilayer construction and has a passage system for the air flow. Between an outer wall which forms a first layer or shell of the sleeve and the inner components of the sleeve there is a first hollow space in which a distribution and a heating of the air, i.e. a cooling of the sleeve, takes place. In the passage system which follows the first hollow space a further heating of the air results. Instead of or in addition to the passages, porous gas permeable parts can also be incorporated in the sleeve which form a dynamic thermal insulation. The air which flows through the pores of the thermal insulation in the radial direction takes up heat which is transmitted from the cell stack principally by thermal radiation and is absorbed by the material of the thermal insulation. The heat taken up is transported by the air back into the cell stack.
The afterburning chambers are formed as axially directed collecting channels through which the exhaust gas is led. The exhaust gas, which is sucked away, flows out of the chambers radially outwardly and subsequently flows on axially. Prior to the passage from the sleeve into the cell stack, the air is heated up further at the outer walls of the afterburning chambers by the heat given up by the exhaust gas which is flowing axially in the chamber, the heat corresponding to the heat arising during the afterburning and part of the heat liberated during the electro-chemical reactions.
The fuel cell battery is used in a plant which is part of a building infra-structure, with the energy converted by the fuel cells being utilized in the form of thermal energy (e.g., for heating purposes) and of electrical energy.
A disadvantage of the known plant with the fuel cell battery is that exhaust gas flowing axially into the afterburning chambers leads to unfavourable temperature gradients in the direction of the stack axis. Moreover, a seal between the stack and the chamber walls is problematic. This problem is a design problem which is associated with differing thermal expansion behavior of the components.